first of all, great idea! thank you!
after trying to follow your guide, I have ended up rebuilding most of the components from scratch due to either Home Assistant deprecating things, or my lack of knowledge/understanding. In order to avoid this type on stressful for anyone else, I have published an up to date guide on my GitHub page: om1d3/ha-purchase-tracker
I am happy if it end up helping anyone.
keep up the good work and thank you again for this great idea.
Horia.